The field of this invention, generally, is heat exchangers, and, more particularly, it deals with the type of condensing and evaporating system referred to in the art as a heat pipe.
While water is a highly desirable heat pipe fluid for operating temperatures between 50.degree. C. and 250.degree. C. because of its high latent heat of vaporization, a severe limitation exists in the potential threat of damage to a water loaded heat pipe, due to freezing of the water.
When a water heat pipe freezes, the expansion resulting as the water changes to ice can cause rupture of the heat pipe casing in much the same way as household plumbing is damaged by freezing.
The freezing problem is particularly serious if a heat pipe freezes when in a vertical or in an inclined position rather than in the horizontal position. In such situations a puddle of water which spans the entire diameter can form at the lower end of the heat pipe, and such a puddle, when frozen, exerts considerable force on the heat pipe wick and casing, frequently causing rupture of the casing.
One approach to solving this problem to date has been the most obvious one, preventing freezing of the liquid. However, in commercial, as opposed to laboratory, operations such precautions are not always feasible, and the actual result has been a reluctance to use freezing prone liquids, such as water, in heat pipes.
A second method of freeze damage prevention is shown in U.S. Pat. Nos. 4,194,559, 956,680 by Eastman. In that patent the quantity of liquid loaded into the heat pipe is limited to the quantity which will be retained in the wick at all times. The puddle at the bottom of the heat pipe therefore never forms, and thus cannot exert destructive forces on the casing.
To date, however, there is no wickless heat pipe or a heat pipe with non-critical fluid fill which will survive repeated freeze-thaw cycles without damage.